Little is known about the mechanisms and pathways for water transport in epithelia such as the renal proximal tubule, the small intestine and the gallbladder. In this project, gallbladders of two species of salamanders (Necturus maculosus and Amphiuma means) will be employed, because of their structural and functional simplicity and large cell size, as model systems for the epithelia listed above. Novel techniques, mostly electrophysiologic, have been and will continue to be developed in order to measure accurately water fluxes across individual barriers, calculate water permeability coefficients, determine the magnitude of the driving forces for water transport, and establish the relationships between water and ion fluxes. Answers to the following main questions will be sought: a) Is water transport transcellular, paracellular, or both? b) What are the hydraulic permeability coefficients of the apical and basolateral cell membranes and of the intercellular pathway? c) What are the driving forces for water transport? In particular, are the osmotic pressure differences large enough to account for the normal rate of water absoption? d) What are the relationships of salt transport and cell volume, and what are the effects of the latter on transmembrane ion fluxes? The methods proposed to answer these questions include microelectrode measurements of cell volume changes from the chemical activity of an impermeant intracellular volume marker, determination of chemical activities of native and/or extraneous ions in the unstirred fluid layers on both sides of the epithelium in the cell interior and in the lateral intercellular spaces, measurements of fluid absorption from salt solution droplets placed over the tissue under oil, and determinations of fluid conductivity with high accuracy and space resolution. The results of these investigations are expected to provide unique, significant information on the mechanisms of salt-coupled water transport in organs such as the kidney, the intestine, the choroid plexus and the gallbladder. In addition, they will improve our understanding of water transport, and its relation to other transport phenomena, in animal cells in general.